It's bad because people still have to ask everyone why it is bad.
Seriously, if threads like these just stopped, we wouldn't have the problem.
The mechanics of Halo are quite lackluster, being rather basic for a first person shooter, however, this is one of the reasons I like the game. The story, while apparently supposed to be deep and shit, didn't appear to be there to me, and the levels were quite well designed and certainly well populated so the shooting part of Halo worked quite well, without being bogged down by story.
Personally, the multiplayer of Halo 2 and 3 wasn't nearly as great as I have it made out to me repeatedly. For multiplayer, I'd go for Team Fortress 2, Modern Warfare or Unreal Tournament depending on my mood. Only Halo 1 has ever got me hooked on its multiplayer, and that's because of how well 'balanced' it was... At least, as long as everyone used human weapons. Especially among friends, Halo was my favourite game to play.
It all comes down to taste and personal preference, I don't understand this die hard war between the fans of Halo and the bashers of it, or any game, or piece of media, or any object. I could argue for hours to a scat fetishist why it's disgusting, but that wouldn't change the fact that they find sexual pleasure out of being crapped on.
Same goes for gamers, although in some cases it seems as though they find being shat on pleasurable from even more than sexual reasons. >_<